A typical inkjet printer reproduces an image by ejecting small drops of ink from a printhead containing nozzles, where the ink drops land on a receiver medium (typically paper) to form ink dots. A typical inkjet printer reproduces a color image by using a set of color inks, usually cyan, magenta, yellow, and black. It is well known in the field of inkjet printing that if ink drops placed at neighboring locations on the page are printed at the same time, then the ink drops tend to flow together on the surface of the page before they soak into the page. This can give the reproduced image an undesirable grainy or noisy appearance often referred to as “coalescence”. It is known that the amount of coalescence present in the printed image is related to the amount of time that elapses between printing adjacent dots. As the time delay between printing adjacent dots increases, the amount of coalescence decreases, thereby improving the image quality. Other image artifacts may arise from printing of adjacent ink droplets in a short period of time. One such artifact is density banding, in which horizontal lines or streaks that vary in density from light to dark appear in the print and degrade the image quality. Another such artifact is gloss banding, in which unwanted variations in the image gloss appear in horizontal bands across the image, degrading the image quality. There are many techniques present in the prior art that describe methods of increasing the time delay between printing adjacent dots to reduce image artifacts using methods referred to as “interlacing”, “print masking”, or “multipass printing”. There are also techniques present in the prior art for reducing one-dimensional periodic artifacts referred to as “bands” or “banding.” This is achieved by advancing the paper by an increment less than the printhead width, so that successive passes or “swaths” of the printhead overlap. The techniques of print masking and swath overlapping are typically combined. See, for example, U.S. Pat. Nos. 4,967,203 and 5,992,962. The term “print masking” generically means printing subsets of the image pixels in multiple partially overlapping passes of the printhead relative to a receiver medium.
There are several techniques known in the prior art for reducing image artifacts related to printing adjacent ink drops that teach modifying the printing method based on some analysis of the digital image that is to be printed. For example, U.S. Pat. No. 4,833,626 to Malcolm discloses a method of changing the printhead velocity for each printed line based on an analysis of the data in the line to maximize printer throughput. A similar technique is disclosed in U.S. Pat. No. 5,326,183 to Kasai, et. al.
U.S. Pat. No. 6,517,177 to Moto, et al, discloses a method for printing an image using slow and fast drying inks. The mixture of the slow and fast drying inks is determined based on a measurement of the ambient temperature as well as an analysis of the image data.
U.S. Pat. No. 6,652,060 to Uchida discloses a method of analyzing image data to determine a time delay before printing the second side of duplex print to prevent ink staining or smudging.
U.S. Pat. No. 7,168,784 to Donovan, et al, discloses a method of analyzing an image to determine light and dark regions and create customized print masks, wherein the print mask controls the dots used to print light regions such that they are printed in a fewer number of passes to reduce graininess.
U.S. Pat. No. 5,714,990 to Courtney, et al, discloses a method of printing using an inkjet printer to avoid smearing artifacts that can occur from a subsequent printed page landing on top of a previously printed page that is sitting in the output tray. If the second page comes along too soon (i.e., before the first page has dried), then smearing artifacts can occur. A delay time between pages is used, or the printhead velocity is reduced to achieve the proper amount of time between pages. Similar techniques are disclosed in U.S. Pat. No. 4,566,014 to Paranjpe et al, and U.S. Pat. No. 4,469,026 to Irwin.
U.S. Patent Application Publication No. US2007/0008364A1 to Ishimoto et. al. discloses a method of avoiding density banding and gloss banding artifacts simultaneously in an image by changing the number of nozzles used to print the image on a region by region basis.
It is also known in the art that the amount of coalescence that occurs in a print is dependent on the amount of ink that is printed. Generally speaking, as the volume of ink printed in a given area increases; the likelihood of coalescence will increase. For this reason, coalescence primarily tends to occur in the darker regions of the image. However, not all images will contain artifact-producing regions, so while printing images in a mode that ensures proper time between adjacent drops will produce good image quality; it will result in a long print time, which is displeasing. Additionally, printing images in a shorter time is more desirable, but may result in poor image quality for some images. Therefore, there is a need for a printing method in which each image can be printed in the shortest amount of time while still delivering good image quality.